What is left behind--quality control in germ cell migration

Cell differentiation, cell proliferation, cell death, and cell migration are tightly controlled during animal development and adult homeostasis. Failure to regulate these processes can result in tumor formation and metastasis. Aberrant cells are therefore often cleared by induction of cell death. Recent work has elucidated the mechanism of elimination of mouse primordial germ cells that fail to migrate properly and highlights the similarity of this mechanism to those governing the same phenomenon in Drosophila. In addition, these studies underscore the different functions a single signaling pathway can have in controlling cell survival, cell proliferation, and cell migration during different phases of development.

Found in translation: A new player in EMT

Epithelial mesenchymal transition (EMT) is a complex process that involves changes in gene expression, cytoskeleton organization, cell adhesion, and extracellular matrix composition. Screening for genes mediating EMT and cancer metastasis, Waerner, Alacakaptan, and colleagues identified ILEI, a cytokine-like protein that plays an essential role in EMT, tumor growth, and late steps of metastasis.

Migration of Zebrafish Primordial Germ Cells: A Role for Myosin Contraction and Cytoplasmic Flow

The molecular and cellular mechanisms governing cell motility and directed migration in response to the chemokine SDF-1 are largely unknown. Here, we demonstrate that zebrafish primordial germ cells whose migration is guided by SDF-1 generate bleb-like protrusions that are powered by cytoplasmic flow. Protrusions are formed at sites of higher levels of free calcium where activation of myosin contraction occurs. Separation of the acto-myosin cortex from the plasma membrane at these sites is followed by a flow of cytoplasm into the forming bleb. We propose that polarized activation of the receptor CXCR4 leads to a rise in free calcium that in turn activates myosin contraction in the part of the cell responding to higher levels of the ligand SDF-1. The biased formation of new protrusions in a particular region of the cell in response to SDF-1 defines the leading edge and the direction of cell migration.

Attraction rules: germ cell migration in zebrafish

The migration of zebrafish primordial germ cell towards the region where the gonad develops is guided by the chemokine SDF-1a. Recent studies show that soon after their specification, the cells undergo a series of morphological alterations before they become motile and are able to respond to attractive cues. As migratory cells, primordial germ cells move towards their target while correcting their path upon exiting a cyclic phase in which morphological cell polarity is lost. In the following stages, the cells gather at specific locations and move as cell clusters towards their final target. In all of these stages, zebrafish germ cells respond as individual cells to alterations in the shape of the sdf-1a expression domain, by directed migration towards their target - the position where the gonad develops.

Transition from non-motile behaviour to directed migration during early PGC development in zebrafish

The migration of zebrafish primordial germ cells (PGCs) is directed by SDF-1a and serves as a model for long-range chemokine-guided cell migration. Whereas the development and migration of zebrafish PGCs have been studied in great detail starting at mid-gastrulation stages when the cells exhibit guided active migration [7-8 hours post fertilization (hpf)], earlier stages have not yet been examined. Here we show that the PGCs acquire competence to respond to the chemokine following discrete maturation steps. Using the promoter of the novel gene askopos and RNA elements of nanos1 to drive GFP expression in PGCs, we found that immediately after their specification (about 3 hpf) PGCs exhibit simple cell shape. This stage is followed by a phase at which the cells assume complex morphology yet they neither change their position nor do they respond to SDF-1a. During the third phase, a transition into a ;migratory stage' occurs as PGCs become responsive to directional cues provided by somatic cells secreting the chemokine SDF-1a. This transition depends on zygotic transcription and on the function of the RNA-binding protein Dead end and is correlated with down regulation of the cell adhesion molecule E-cadherin. These distinctive morphological and molecular alterations could represent a general occurrence in similar processes critical for development and disease.

CXCR4 and Gab1 cooperate to control the development of migrating muscle progenitor cells

Long-range migrating progenitor cells generate hypaxial muscle, for instance the muscle of the limbs, hypoglossal cord, and diaphragm. We show here that migrating muscle progenitors express the chemokine receptor CXCR4. The corresponding ligand, SDF1, is expressed in limb and branchial arch mesenchyme; i.e., along the routes and at the targets of the migratory cells. Ectopic application of SDF1 in the chick limb attracts muscle progenitor cells. In CXCR4 mutant mice, the number of muscle progenitors that colonize the anlage of the tongue and the dorsal limb was reduced. Changes in the distribution of the muscle progenitor cells were accompanied by increased apoptosis, indicating that CXCR4 signals provide not only attractive cues but also control survival. Gab1 encodes an adaptor protein that transduces signals elicited by tyrosine kinase receptors, for instance the c-Met receptor, and plays a role in the migration of muscle progenitor cells. We found that CXCR4 and Gab1 interact genetically. For instance, muscle progenitors do not reach the anlage of the tongue in CXCR4;Gab1 double mutants; this target is colonized in either of the single mutants. Our analysis reveals a role of SDF1/CXCR4 signaling in the development of migrating muscle progenitors and shows that a threshold number of progenitor cells is required to generate muscle of appropriate size.

Germ Cells: Sex and Repression in Mice

The mouse Blimp1 gene encodes a transcriptional repressor that is essential for B-cell development. Recent studies have shown that the Blimp1 protein also plays a critical role in the specification of mouse primordial germ cells.

Development without germ cells: the role of the germ line in zebrafish sex differentiation

The progenitors of the gametes, the primordial germ cells (PGCs) are typically specified early in the development in positions, which are distinct from the gonad. These cells then migrate toward the gonad where they differentiate into sperms and eggs. Here, we study the role of the germ cells in somatic development and particularly the role of the germ line in the sex differentiation in zebrafish. To this end, we ablated the germ cells using two independent methods and followed the development of the experimental fish. First, PGCs were ablated by knocking down the function of dead end, a gene important for the survival of this lineage. Second, a method to eliminate the PGCs using the toxin-antitoxin components of the parD bacterial genetic system was used. Specifically, we expressed a bacterial toxin Kid preferentially in the PGCs and at the same time protected somatic cells by uniformly expressing the specific antidote Kis. Our results demonstrate an unexpected role for the germ line in promoting female development because PGC-ablated fish invariably developed as males.

Primordial germ cell migration in the chick and mouse embryo: the role of the chemokine SDF-1/CXCL12

As in many other animals, the primordial germ cells (PGCs) in avian and reptile embryos are specified in positions distinct from the positions where they differentiate into sperm and egg. Unlike in other organism however, in these embryos, the PGCs use the vascular system as a vehicle to transport them to the region of the gonad where they exit the blood vessels and reach their target. To determine the molecular mechanisms governing PGC migration in these species, we have investigated the role of the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) in guiding the cells towards their target in the chick embryo. We show that sdf-1 mRNA is expressed in locations where PGCs are found and towards which they migrate at the time they leave the blood vessels. Ectopically expressed chicken SDF-1alpha led to accumulation of PGCs at those positions. This analysis, as well as analysis of gene expression and PGC behavior in the mouse embryo, suggest that in both organisms, SDF-1 functions during the second phase of PGC migration, and not at earlier phases. These findings suggest that SDF-1 is required for the PGCs to execute the final migration steps as they transmigrate through the blood vessel endothelium of the chick or the gut epithelium of the mouse.

Signaling pathways controlling primordial germ cell migration in zebrafish

During their migration, zebrafish primordial germ cells (PGCs) rely on directional cues provided by the chemokine SDF-1a, whose receptor is CXCR4b. The molecular mechanisms whereby CXCR4b activation is interpreted intracellularly into directional migration are not known. Here we investigate the role of two important biochemical pathways -- G-protein-dependent and phosphoinositide 3-kinase (PI3K)-dependent signaling -- in directing PGC migration in zebrafish. We show that G proteins of the Gi family are essential for directional migration but not for PGC motility. Inhibition of PI3K signaling in PGCs slows down their migration and leads to abnormal cell morphology as well as to reduced stability of filopodia. Invariably, during directed PGC migration, the distribution of the products of PI3K activity - phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] and/or phosphatidylinositol (3,4)bisphosphate [PtdIns(3,4)P(2)] -- is not polarized, and reducing the level of these 3-phosphoinositides does not affect the ability of PGCs to migrate directionally. We therefore conclude that Gi-dependent signaling is essential for directional migration, whereas the PI3K pathway is important for the actual motility of PGCs.

Autonomous modes of behavior in primordial germ cell migration

Zebrafish primordial germ cells (PGCs) are guided toward their targets by the chemokine SDF-1a. PGCs were followed during three phases of their migration: when migrating as individual cells, while remaining in a clustered configuration, and when moving as a cell cluster within the embryo. We found that individually migrating PGCs alternate between migratory and pausing modes. Pausing intervals are characterized by loss of cell polarity and correlate with subsequent changes in the direction of migration. These properties constitute an intrinsic behavior of PGCs, enabling erasure of prior polarity and re-sampling of the environment. Following migration arrest at a site of high SDF-1a levels, PGCs resume migration as a cluster. The seemingly coordinated cluster migration is a result of single-cell movement in response to local variations in SDF-1a distribution. Together, these behavioral modes allow the cells to arrive at specific destinations with high fidelity and remain at their target site.

Involvement of Pax6 and Otx2 in the forebrain-specific regulation of the vertebrate homeobox gene ANF/Hesx1

During early vertebrate development, ANF homeobox genes are expressed in the prospective forebrain. Their regulation is essential for correct morphogenesis and function of the prosencephalon. We identified a 1-kb fragment upstream of the chicken GANF gene sufficient to drive lacZ expression in the endogenous expression domain. Concordant with the high conservation of this sequence in five investigated species, this element is also active in the corresponding expression domain of the zebrafish orthologue. In vivo analysis of two in vitro-identified Otx2 binding sites in this conserved sequence revealed their necessity for activation of the chicken ANF promoter. In addition, we identified a Pax6-binding site close to the transcriptional start site that is occupied in vivo by Pax6 protein. Pax6 and GANF exhibit mutually exclusive expression domains in the anterior embryonic region. Overexpression of Pax6 in chick embryos inhibited the endogenous GANF expression, and in Pax6(-/-) mice the expression domain of the murine ANF orthologue Hesx1 was expanded and sustained, indicating inhibitory effects of Pax6 on GANF. However, a mutation of the Pax6 site did not abolish reporter activity from an electroporated vector. We conclude that Otx2 and Pax6 are key molecules involved in conserved mechanisms of ANF gene regulation.

Guidance of primordial germ cell migration

Primordial germ cells (PGCs), the progenitors of the gametes, migrate from the position where they are specified towards the region where the gonad develops. To reach their target, the PGCs obtain directional cues from cells positioned along their migration path. One such cue, the chemokine SDF-1, has recently been found to be critical for proper PGC migration in zebrafish and in mice. In Drosophila, too, a molecule that is structurally related to chemokine receptors and is important for PGC migration has been identified. The ability to visualize chemokine-guided migration at a high resolution in vivo in these model organisms provides a unique opportunity to study this process, which is relevant for many events in normal development and disease.

Immunostimulatory DNA sequences : an overview

The biochemical and genetic properties of DNA have been thoroughly investigated, yet only recently has it been appreciated that DNA carries more information than simply a blueprint for the regulation and construction of proteins. Indeed, the immune systems of vertebrates appear to have evolved the ability to distinguish the foreign DNA of bacteria and certain viruses from the self-DNA of the host, a new twist on the self vs non-self detection system already well-known for foreign proteins. Specifically, the frequency of unmethylated CpG motifs (CpG denotes covalently linked CG dinucleotides, not C∶G base pairs) is extensively suppressed in vertebrates, including mammals (by at least 20-fold [1]), whereas it is found at the usual frequency (1/16) in most bacterial and viral DNA. There have now been several reports (detailed in Subheadings 2. and 4. ) that bacterial DNA or synthetic oligodeoxyribo-nucleotides (ODNs) containing bacterially derived sequences, stimulate the immune systems of mice and humans to first mount innate, and then antigen-specific (when foreign antigen is present), Th(1)-type responses. This adjuvant effect of bacterial immunostimulatory DNA sequences (ISS) appears to be important for the robust Th(1)-type immune response usually seen in genetic vaccination (2). Although the terms CpG motif and ISS are generally used synonymously in this field, CpG motifs are defined structurally, whereas ISS are defined functionally (and therefore include non-CpG sequences that have been found to be stimulatory).

The chemokine SDF1/CXCL12 and its receptor CXCR4 regulate mouse germ cell migration and survival

In mouse embryos, germ cells arise during gastrulation and migrate to the early gonad. First, they emerge from the primitive streak into the region of the endoderm that forms the hindgut. Later in development, a second phase of migration takes place in which they migrate out of the gut to the genital ridges. There, they co-assemble with somatic cells to form the gonad. In vitro studies in the mouse, and genetic studies in other organisms, suggest that at least part of this process is in response to secreted signals from other tissues. Recent genetic evidence in zebrafish has shown that the interaction between stromal cell-derived factor 1 (SDF1) and its G-protein-coupled receptor CXCR4, already known to control many types of normal and pathological cell migrations, is also required for the normal migration of primordial germ cells. We show that in the mouse, germ cell migration and survival requires the SDF1/CXCR4 interaction. First, migrating germ cells express CXCR4, whilst the body wall mesenchyme and genital ridges express the ligand SDF1. Second, the addition of exogenous SDF1 to living embryo cultures causes aberrant germ cell migration from the gut. Third, germ cells in embryos carrying targeted mutations in CXCR4 do not colonize the gonad normally. However, at earlier stages in the hindgut, germ cells are unaffected in CXCR4(-/-) embryos. Germ cell counts at different stages suggest that SDF1/CXCR4 interaction also mediates germ cell survival. These results show that the SDF1/CXCR4 interaction is specifically required for the colonization of the gonads by primordial germ cells, but not for earlier stages in germ cell migration. This demonstrates a high degree of evolutionary conservation of part of the mechanism, but also an area of evolutionary divergence.

Primordial germ-cell development: the zebrafish perspective

Primordial germ cells follow a characteristic developmental path that is manifested in the specialized regulation of basic cell functions and behaviour. Recent studies in zebrafish have greatly enhanced our understanding of the mode of specification of primordial germ cells, cell-fate maintenance and the migration of these cells towards their target, the gonad, where they differentiate into gametes.

dead end, a novel vertebrate germ plasm component, is required for zebrafish primordial germ cell migration and survival

In most animals, primordial germ cell (PGC) specification and development depend on maternally provided cytoplasmic determinants that constitute the so-called germ plasm. Little is known about the role of germ plasm in vertebrate germ cell development, and its molecular mode of action remains elusive. While PGC specification in mammals occurs via different mechanisms, several germ plasm components required for early PGC development in lower organisms are expressed in mammalian germ cells after their migration to the gonad and are involved in gametogenesis. Here we show that the RNA of dead end, encoding a novel putative RNA binding protein, is a component of the germ plasm in zebrafish and is specifically expressed in PGCs throughout embryogenesis; Dead End protein is localized to perinuclear germ granules within PGCs. Knockdown of dead end blocks confinement of PGCs to the deep blastoderm shortly after their specification and results in failure of PGCs to exhibit motile behavior and to actively migrate thereafter. PGCs subsequently die, while somatic development is not effected. We have identified dead end orthologs in other vertebrates including Xenopus, mouse, and chick, where they are expressed in germ plasm and germ-line cells, suggesting a role in germ-line development in these organisms as well.

Immunostimulatory DNA-based therapeutics for experimental and clinical allergy

Although allergen immunotherapy (IT) has been shown to modulate allergic hypersensitivities, its efficacy is limited. Recently, in various models of experimental allergy, a number of reagents which we have termed immunostimulatory DNA-based therapeutics have proven highly effective in both the prevention and reversal of Th2 mediated hypersensitivity states. These include immunization with gene vaccines, allergen mixed with immunostimulatory oligodeoxynucleotide (ISS-ODN), and physical allergen: ISS-conjugates (AIC), and immunomodulation with ISS-ODN alone. Results from our laboratory have shown that immunostimulatory DNA-based therapeutics may be effective for the reversal of allergic hypersensitivity states in humans and several clinical trials have already been initiated. This review will focus on our laboratory's experience with immunostimulatory DNA-based therapeutics in various murine models of allergy and their potential utility in the treatment of allergic patients.

Primordial germ cell development in zebrafish

In sexually reproducing organisms, primordial germ cells (PGCs) give rise to gametes that are responsible for the development of a new organism in the next generation. These cells follow a characteristic developmental path that is manifested in specialized regulation of basic cell functions and behavior making them an attractive system for studying cell fate specification, differentiation and migration. This review summarizes studies aimed at understanding the development of this cell population in zebrafish and compares these results with those obtained in other model organisms.

Guidance of primordial germ cell migration by the chemokine SDF-1

The signals directing primordial germ cell (PGC) migration in vertebrates are largely unknown. We demonstrate that sdf-1 mRNA is expressed in locations where PGCs are found and toward which they migrate in wild-type as well as in mutant embryos in which PGC migration is abnormal. Knocking down SDF-1 or its receptor CXCR4 results in severe defects in PGC migration. Specifically, PGCs that do not receive the SDF-1 signal exhibit lack of directional movement toward their target and arrive at ectopic positions within the embryo. Finally, we show that the PGCs can be attracted toward an ectopic source of the chemokine, strongly suggesting that this molecule provides a key directional cue for the PGCs.

Production of maternal-zygotic mutant zebrafish by germ-line replacement

We report a generally applicable strategy for transferring zygotic lethal mutations through the zebrafish germ line. By using a morpholino oligonucleotide that blocks primordial germ cell (PGC) development, we generate embryos devoid of endogenous PGCs to serve as hosts for the transplantation of germ cells derived from homozygous mutant donors. Successful transfers are identified by the localization of specifically labeled donor PGCs to the region of the developing gonad in chimeric embryos. This strategy, which results in the complete replacement of the host germ line with donor PGCs, was validated by the generation of maternal and maternal-zygotic mutants for the miles apart locus. This germ-line replacement technique provides a powerful tool for studying the maternal effects of zygotic lethal mutations. Furthermore, the ability to generate large clutches of purely mutant embryos will greatly facilitate embryological, genetic, genomic, and biochemical studies.